1. Field of the Invention
The present invention relates to a wheel disk for an automobile.
2. Description of Related Art
As illustrated in FIG. 7, a wheel is a welded assembly of a dish-like disk 1 and an annular rim 2. A disk for a large-sized wheel such as wheels for busses and trucks, made according to the process of FIG. 8, is manufactured by the steps of die-cutting a circular blank 4 from a square flat plate 3 (step 2), drawing the blank 4 into the form of a disk configuration by a press (step 3), perforating a hub hole and decoration holes by a press (step 4), and drilling nut holes by a multiple drill and machining nut seats (step 5). The thus constructed disk manufactured from a flat plate has a substantially uniform thickness as shown in FIG. 7.
Alternatively, a disk may be formed to a disk configuration through cold spinning so as to have a thickness gradually decreased from a hub coupling portion to a disk end portion. In the case of a gradually changing thickness, it has been thought to be preferable that the thickness of the disk end portion should be decreased to one half of the original thickness of the plate.
However, according to a stress analysis of a disk wheel conducted using a finite element method, it is clear that the stress distributions of the conventional disk having a substantially uniform thickness and the conventional disk having a gradually decreased thickness are not ideal. As a result, a relatively large stress concentration is caused at a weld portion connecting the disk end portion and the rim, and a portion of the disk is thicker than needed to be accompanied by an increase in weight.